Endodontia is a branch of dentistry specializing in diseases of the tooth pulp. A primary corrective procedure for diseased pulp is to remove it, clean out and shape the resulting root canal and then obturate the root canal space. The obturation procedure is critical because if the apical seal is not adequate, the tissues underlying the root canal may be exposed to foreign, deleterious matter.
Obturation of the root canal system typically involves the insertion of a material within the root canal, which material must sealably adhere to the dentin walls. Of particular concern is a fluid-tight seal between the inserted material and the apical foramen region. Additionally, endodontic therapy requires that the inserted material conform, and effect a seal, to the dentin wall irregularities as well as any lateral canals. Various methodologies for obturating root canals have been developed and are disclosed in the prior art. A thorough understanding of these procedures is necessary to appreciate the significance and novelty of the present invention.
While many types of obturating material have been used, modern endodontia techniques utilize trans-polyisoprenes such as gutta-percha and balata or other species thereof. In order to avoid a repetitively detailed identification of the trans-polyisoprenes utilized, the specification will hereinafter simply refer to gutta-percha. Gutta-percha, chemically, is the trans- isomer of natural rubber and is a tough, crystalline, thermoplastic polymer. The thermoplastic characteristic of gutta-percha makes it a particularly useful endodontia material, because in the plasticized state gutta-percha readily adapts to the contour of the dentin walls and tends to retain its shape after cooling, albeit with some shrinkage.
Verification of the effectiveness of a particular obturation technique may be performed in vitro using dye penetrant, radiotracer penetrant, microscopic examination, sectioning, X-ray analysis or scanning electron microscope analysis, among others. Though in vivo assessment is possible, a much greater period of time is required before the results are available. The particular analysis technique utilized will, in part, be determined by which aspect of the obturation seal may be of interest, as well known and understood by those skilled in the art.
One obturation technique involves the use of gutta-percha cones, or points, usually made in standard sizes, a plurality of which are fitted into the root canal which has itself been generally conically shaped to facilitate acceptance of the cones. After the canal has been cleaned and shaped, the dentin walls are typically coated with a sealer and a point is inserted. The inserted point is then condensed with finger pluggers to force the tip of the point into conformation with the apical region of the root canal. With this technique the gutta-percha point typically is not heated, and a second step, lateral condensation, is required. Lateral condensation is effected by inserting additional points into the canal and compacting them with heated finger pluggers to force the point material into lateral conformity with the canal wall, and hopefully to conform the point material not only to the irregularities therein but also to any laterally extending canals. Numerous difficulties can arise with this method. A major inconvenience is having to impart a particular shape to the apex of the canal in order that it can accept a standard gutta-percha point. Irregularities in the dentin walls may allow for a less than satisfactory adaptation of the point to the canal cavity. Also, the sealer may not be uniformly distributed within the interface between the point and the dentin walls.
Another obturation technique is generally referred to as vertical condensation and utilizes heated gutta-percha. The canal is prepared and a sealer introduced, as previously described. The gutta-percha to be introduced into the canal, however, is first heated. A plurality of warmed gutta-percha segments may be compactably inserted into the canal chamber. Heating the gutta-percha reduces its viscosity and thereby allows the material more readily to adapt to the dentin walls than unheated gutta-percha. Unexpectedly, however, the degree of lateral condensation, or conformity, has been found to be reduced.
When employing this technique, it has been found that the material extruded into lateral canals consists primarily of sealer. As anticipated, fillings made by vertical condensation techniques show close adaptation of the gutta-percha to the dentin walls. However, voids are often seen, possibly due to reduced lateral condensation, and the root canal sealer is not always present at the interface. The gutta-percha also tends to cool quickly, particularly during the considerable time required for the vertical condensation technique. Once the material cools, viscosity increases and lateral flow is substantially reduced. Also, seams have been observed which may be indicative of incomplete conjoining of different gutta-percha segments.
A third technique uses a solvent such as chloroform to soften the gutta-percha. While this technique shows good adaptation in the apical region, voids are often seen coronal to this section. The surface of the filling is often wrinkled which is most probably due to shrinkage of the material. Such shrinkage is an undesirable feature as it may reduce the integrity of the required seal between the filling material and the obturated canal. Shrinkage may also occur in the vertical condensation technique during cooling.
The use of plasticizers or other additives in the nature of processing aids has been considered, but there are three major drawbacks. First, it would require lengthy governmental testing and subsequent approval to assure that the processing additives are not deleterious for use in endodontic therapy. Second, even though such approval might be acquired there would be a reluctance on the part of the majority of practitioners to accept a new product without considerable clinical evidence of its satisfactory performance without incident. Third, processing additives appear to create unfavorable shrinkage characteristics which would work to destroy the seal required to obturate a root canal.
Although the above obturation techniques--with perhaps the noted exception as to the use of additives--generally provide reasonably acceptable results when performed properly, the methods are time consuming, and, therefore, costly, and require considerable care to effectuate a fluid-tight seal, particularly at the apical foramen.
One of the most promising obturation techniques is generally referred to as thermoplastic injection of a polymeric material such as gutta-percha. The important feature of this technique is that the polymer is heated to its molten, or plasticized, state--typically about +160.degree. C. The polymer is then forced, under mechanically generated pressure, into the root canal system.
One of the earlier methods for in vitro "thermoplastic injection" is described by Yee, et al, Three-dimensional Obturation of the Root Canal Using Injection-Molded, Thermoplasticized Dental Gutta-Percha, JOURNAL OF ENDODONTICS, Vol. 3, No. 5, May, 1977. The root canal system is shaped and cleaned using conventional techniques. The gutta-percha is next introduced into the root canal cavity using an endodontic pressure syringe. An 18-gauge needle may typically be used inasmuch as it is the maximum size that fits conveniently into the root canal in human anterior teeth.
To prepare for the injection technique, gutta-percha cones are manually loaded into a syringe, and the barrel, with the needle attached, is then heated in a glycerine bath until an unrestrained flow can be achieved. This occurs at about +160.degree. C. The needle is then inserted into the cavity and the gutta-percha extruded therethrough to fill the cavity. When a slight resistance from the injected material is felt, the needle is withdrawn a few millimeters coronally and more material is extruded. This process continues until the canal is completely obturated.
Analysis of the results of obturation by this technique shows few voids and excellent adaptation of the polymer to the dentin walls. A particular advantage is that the plasticized polymer flows both vertically and laterally. Moreover, the sealer tends to be evenly distributed and lateral canals can be effectively obturated.
While the above-described method is useful for in vitro analysis of injection-molded thermoplasticized polymers, clinical application is severely limited due to the excessively high temperatures required to plasticize gutta-percha and similar polymers. These temperatures make handling of the applicator and working in the periapical and oral regions somewhat difficult and possibly unacceptable because of the fear in the mind of the clinician that its use could, even remotely, be injurious to the patient.
In order to circumvent the high temperature related problems which make the injection process clinically undesirable and impractical, much work has been dedicated to developing more feasible delivery systems. One such system is described by Marlin, et al, Clinical Use of Injection-Molded Thermoplasticized Gutta Percha for Obturation of the Root Canal System: A Preliminary Report, JOURNAL OF ENDODONTICS, Vol. 7, No. 6, June, 1981. The equipment includes an injection syringe and an electrical heating unit. The barrel of the syringe carries an electrical heating element and is insulated not only to minimize heat dissipation but also to protect the clinician and patient. The level of heat is variable depending, in part, on the needle gauge. Standard gutta-percha points are loaded into the syringe, plasticized, and the gutta-percha is then inserted in a manner similar to the Yee, et al in vitro technique hereinbefore described.
While this method of delivery is an improvement on some aspects of prior known systems, it does necessitate the use of a considerably more complex and expensive delivery system. It must be understood, however, that this method is only a delivery system; it is not a filling system. Tests have revealed that by this procedure the plasticized gutta-percha is delivered only one-half (1/2) the distance between the tip of the injection needle and the apex of the canal. Further manipulation, as with finger pluggers, is required to assure that the canal is completely filled. Moreover, the high temperatures at which the gutta-percha must be delivered by this procedure appears, itself, to engender two major drawbacks. First, mental unacceptability--the clinician remains fearful of injecting material at such high temperature into a human patient. Second, questionable operability--it appears that the high temperature differential between the injected gutta-percha and its surrounding environment likely results in excessive shrinkage of the gutta-percha as it cools.
Another method is recounted by Lugassy, et al, Root Canal Obturation with Gutta-Percha: A Scanning Electron Microscope Comparison of Vertical Compaction and Automated Thermatic Condensation, JOURNAL OF ENDODONTICS, Vol. 8, No. 3, March, 1982. The technique was developed by McSpadden and is generally referred to as automated thermatic condensation. This technique uses a compactor, similar to a Hedstroem file, mounted on a contra-angle. The compactor plasticizes the gutta-percha within the root canal system and provides lateral as well as vertical compaction.
According to this technique, after the root canal is shaped and prepared, a compactor size is selected one size smaller than the largest reamer used near the apical constriction. A standard gutta-percha point is inserted into the canal, and the compactor is then rotated at a speed of approximately 10,000-15,000 r.p.m. The direction of rotation must be such as to assure an apical vector for the gutta-percha compaction. Essentially, the rotary tool provides frictional heat whereby the gutta-percha is plasticized and adequate lateral and vertical condensation is achieved.
While the McSpadden technique is an improved method for avoiding the excessive heat problem, the technique still requires sophisticated hardware, consummate skill and strict adherence to specific guidelines to obtain predicted results.
It is clear, therefore, that a viable need exists for an improvement in the art of endodontia whereby thermoplastic polymers can be quickly and inexpensively injected with a minimal risk from excessive temperatures and without elaborate delivery systems.